The present invention relates to a composition comprising an aromatic liquid crystalline polymer (LCP) and a low molecular weight polyethylene (PE) which acts as a lubricant.
Thermotropic liquid crystalline polymers are useful as molding resins for a variety of applications. This is due to the desirable properties that many of these polymers possess, such as good moldability, low flammability, high temperature resistance, good physical properties, etc. For electrical applications, such as electrical connectors, low flammability or resistance to burning is important. Further, in many electrical applications, the parts are intricate and have thin walls. As such, these parts are sometimes difficult to eject from the mold. Internal lubricants (sometimes called mold release agents) are useful to aid in the ejection of such parts from the mold. For a discussion of such lubricants, see H. Mark, et al., Ed., Encyclopedia of Polymer Science and Engineering, Vol. 14, John Wiley and Sons, New York, 1988, p. 411-421.
While internal lubricants are generally desirable in many applications, it is preferable that the desired properties of the LCP are degraded as little as possible. It has been surprisingly found that low molecular weight polyethylene (often called a xe2x80x9cwaxxe2x80x9d) which by itself bums quite readily, when added to an LCP as a lubricant, does not significantly degrade the low flammability properties of the LCP.
This invention relates to a composition comprising: a) an aromatic liquid crystalline polymer, and b) from about 0.05 to about 2.5 percent by weight of said aromatic liquid crystaline polymer of polyethylene, said polyethylene having a molecular weight within the range between about 1,000 to about 10,000.
The LCPs useful herein are aromatic LCPs. By an xe2x80x9caromaticxe2x80x9d LCP is meant that the backbone of the polymer is composed of aromatic rings and functional groups such as esters, amides and imides. There are no aliphatic or cycloaliphatic groups which are part of the main chain. However, such groups may be substituted onto the main chain as side groups. For instance, in an LCP which is at least partially a polyester, repeat units may be derived from t-butylhydroquinone, methylhydroquinone, phenethyihydroquinone, methylterephthic acid, t-butyl-4-hydroxybenzoic acid, etc.
Preferred aromatic LCPs are polyesters, poly(imide-esters) and poly(amide-esters), with polyesters being especially preferred. These types of polymers are well known to the artisan, see for instane U.S. Pat. Nos. 4,900,804, 4,851,497, 4,067,852, 4,083,832, 4,727,131, 4,727,129, 4,762,907, 4,664,972, 4,118,372, 4,684,712, 4,522,974, 4,473,682 4,161,470 and 5,110,896. Preferred LCPs may contain repeat units derived from one or more of 4-hydroxybenzoic acid, 6-hydroxynapthoic acid, terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid, 4,4xe2x80x2-bibenzoic acid, hydroquinone, 4,4-biphenol, resorcinol, 2,6-dihydroxynaphthalene, substituted hydroquinones including chlorohydroquinone, methylhydroquinone, t-butylhydroquinone, phenylhydroquinone, and phenethylhydroquinone. Especially preferred polymers include polymers consisting essentially of: 
wherein the molar ratio of(I):(II) ranges from 0:100 to 100:0, wherein the molar ratio of (III):(IV) ranges from 85:15 to 15:85, wherein the molar ratio of the total of (I) and (II) to the total of (III) and (IV) is substantially 1:1, and further wherein there are 100 to 600 moles of (V) per 100 moles of (I) plus (II);
or polymers consisting essentially of the following repeat units: 
wherein the molar ratio of (VI):(VIl) ranges from 0:100 to 100:0, the molar ratio of (VIII):(IX) rages from 25:75 to 90:10, the molar ratio of the sum of (VI) and (VII) to the sum of (VIll) and (IX) is substantially 1:1, the molar ratio of (X):XI) ranges from 97:3 to 50:50, and the number of moles of (X) plus (XI) ranges from 100 to 600 per moles of (VI) plus (VII).
Especially preferred polymers also include copolymers of p-hydroxybenzoic acid and 6-hydroxynapthoic acid.
The polyethylene (xe2x80x9cPExe2x80x9d) useful herein has a molecular weight within the range between about 1,000 to about 10,000, and preferably about 1,500 to about 4,000. Low pressure polymerized (high density), polar PE is preferred for use in the present invention. Such PEs are made by oxidation of PE which introduces polar functional groups, such as carboxyl groups, into the PE. Such oxidized PEs are commercially available, such as those from Hoechst Celanese Corp., Somerville, N.J., U.S.A. under the designation xe2x80x9cHoechst Wax PED.xe2x80x9d
The amount of PE present in the present invention generally ranges from about 0.05 to about 2.5 percent by weight of the LCP, preferably about 0.1 to about 2.0 percent by weight, and most preferably about 0.2 to about 1.0 percent by weight. Other materials usually found in LCP molding compositions may also be present, such as fillers, including glass, clay, talc and other minerals, and carbon black, pigments, antioxidants, other polymers, etc. Preferred fillers are glass fiber, talc, titanium dioxide and carbon black, and an especially preferred filler is glass fiber.
Flammability or flame resistance properties reported herein were measured by UL-94, revised as of May 14, 1993, and published by Underwriters Laboratories, Inc., Melville, N.Y., U.S.A. It is preferred that the flammability (as measured by the total after flame time for any condition set) of the LCP compositions not containing PE is increased by less than 50% when the PE is added, more preferably less than 25%, and especially preferably increased less than 10%.
It is also preferred that the overall UL-94 rating under the particular test conditions used is the same or better (less flammable rating) for the PE containing LCP composition compared to the LCP composition without PE. It is particularly preferred if the PE containing LCP composition still retains a UL-94 rating of V-0.
The PE containing LCP compositions claimed herein may be made by standard methods. For instance these materials (and others to be in the composition) may be mixed together in a single or twin screw extruder. The composition may be then directly melt formed or formed into pellets for later use. Melt forming may be done by extrusion or injection molding, for example, and the latter is preferred. Such formed parts are useful as films, or as molded parts in the automotive, electronic or aerospace applications. As mentioned above, the composition is particularly usefull for electronic parts, particularly electronic connectors.
In the Examples, the starting LCP used in had a composition of: (structures shown above, in mole percent) (I), 9.6%; (II), 9.6%; (III), 13.5%; (IV), 5.8%; and (V), 61.5% (see U.S. Pat. No. 5,110,896). The fiberglass used in the Examples was grade 408 from Owens-Corning Fiberglass Corp. The polyethylene was reported to be an oxidized low pressure polyethylene, from Hoechst Celanese Corp., Somerville, N.J., U.S.A. as grade PED 521. It reportedly had a drop point of about 105xc2x0 C., an acid value of about 35 mg KOH/g and a density of about 0.95 at 20xc2x0 C., using test methods specified by Hoechst Celanese Corp.
The materials were mixed on a Werner and Pfleiderer 28 mm twin screw extruder, in which the zone temperatures (rear to front) were approximately 255xc2x0 C., 290xc2x0 C., 290xc2x0 C., 290xc2x0 C., 300xc2x0 C. and 345xc2x0 C. (die). A one-hole die was used, and pellets were made. The pellets were then injection molded into test bars for the Flammability test with temperatures in the injection molding machine (rear to front) being approximately 325xc2x0 C., 320xc2x0 C., 330xc2x0 C., 335xc2x0 C., and 347 xc2x0 C.
Mold-release Test